The inlet ductwork leading from the surface of an aircraft to the inlet of an aeronautical auxiliary power unit (APU) is subject to accumulating snow and ice under winter conditions. The snow and ice tend to gather within the vicinity of the APU “inlet muff”. The inlet muff is a duct that is shaped to receive air from an inlet duct that typically has a rectangular cross-section. The muff receives and distributes ambient air from the inlet duct around the periphery of the inlet to the APU. The APU inlet is commonly defined by a ring shaped perforated cylinder that passes the air whilst providing APU structure protection from objects being drawn into the engine. The muff itself has an open side, typically located above the APU engine centreline. The flow into the muff is generally in the downward direction.
Some quantity of ice and snow tends to accumulate on the APU inlet protection screen near the muff inlet, but a greater quantity tends to accumulate on the side opposite the muff inlet, near the bottom of the muff. The bottom of the muff usually has a “gull-wing” fairing that avoids stagnating the airflow and inhibits the inlet air from oscillating from side to side within the muff.
The greater quantity of ice and snow that accumulates near the bottom of the muff is difficult to remove. Accumulation of ice and snow in this location can build up to the point that it obstructs air from entering the APU. This condition leads to inlet pressure loss that can build up to the point of compromising the APU power capability.